A heater incorporating a plate-like heating element made of a PTC thermistor is conventionally known. The PTC thermistor is a heating element having a positive temperature coefficient of resistance and, for example, is produced from a PTC material such as semiconductor ceramics of a barium titanate system. The PTC thermistor has low resistance at temperatures ranging from room temperature to Curie temperature Tc (resistance transition temperature) and a rapid increase in the resistance when the temperature exceeds the Curie temperature Tc. With this characteristic, when a voltage is applied to the heating element, the heating element draws high currents initially as the resistance is low at low temperatures, resulting in a rapid increase in the temperature. On the other hand, the temperature of the heating element does not exceed a predetermined temperature because the resistance increases rapidly when the temperature exceeds the Curie temperature Tc. Thus, the heating element constantly maintains the predetermined temperature. Namely, the heating element including the PTC thermistor has self-controlling temperature characteristics. Accordingly, there is no need for a heater with such a heating element to have circuits for controlling the heated temperature to be a predetermined temperature and for preventing overheating. Additionally, such a heater is very safety.
A heater of this type is disclosed in Japanese Publication for Examined Utility Model Application No. 26226/1972. The structure of the heater is as follows. Electrodes are formed on the upper and lower surfaces of a plate-like heating element made of a PTC thermistor. A terminal board is mounted on the outer surface of each electrode, a heat transfer board is mounted on the outer surface of one of the terminal boards, and an electrical insulating board is mounted on the outer surface of the other terminal board. Japanese Publication for Unexamined Utility Model Application No. 53498/1983 also discloses a heater of this type. The heater of this document is constructed such that electrodes are formed on the upper and lower surfaces of a plate-like heating element made of a PTC thermistor, an electrode board or a terminal board is mounted on the outer surface of each electrode, and an electrical insulating board is mounted on the outer surface of each terminal board. In these heaters, lead wires as feeders are connected to the terminal boards by, for example, a solder. Electrical power is supplied to the heating element by connecting the lead wires to a power supply.
However, these structures fail to provide sufficient insulation, and therefore the safety of the heaters drops, particularly, in a high humidity environment.
Japanese Publication for Examined Utility Model Application No. 9283/1991 discloses a heater which solves such problems. In this heater, electrodes are formed on the upper and lower surfaces of a plate-like heating element made of a PTC thermistor, and lead wires are soldered to the outer surfaces of the electrodes. And, the heating element, the electrodes and the connections of the lead wires and the electrodes are coated with a heat-conductive electrical-insulating resin, such as a silicone resin.
With this structure, satisfactory insulation is achieved. However, since the lead wires are soldered to the outer faces of the electrodes, the heat conductive insulating resin coat is needed to have a thickness which covers up bumps on the electrode surfaces caused by the soldering of the lead wires, resulting in an increase in the thickness of the resin layer. Consequently, the heater with this structure becomes rather thicker and larger, but is not capable of efficiently conducting heat from the heating element to an object to be heated.
In addition, when the heater incorporating a PTC thermistor as a heating element is used, if the heating element is not coated well or the insulation structure is not appropriate, the electrical characteristics may deteriorate, causing electrical insulation defect and variations in the electric resistance. Such deterioration is caused by dust and humidity in the atmosphere. In particular, when dew is formed on the electrode surface of the heating element, that moisture causes an electrical chemical reaction on the electrode surface upon the application of a voltage. This may cause the electric resistance to vary considerably. In order to solve such a problem, Japanese Publication for Examined Patent Application No. 47500/1978 and Examined Utility Model Application No. 9283/1991 disclose heating elements covered with an electrical insulating cover member such as a resin material.
For instance, with a covering method disclosed in the above Japanese Examined Patent Application No. 47500/1978, a heating unit formed by connecting lead wires to a heating element is covered with an electrical insulating cover member. In this case, in order to position the heating unit more easily and properly in the electrical insulating cover member formed by molding, the covering is performed through the following processes.
Firstly, a plastic pot having an open top and a base with holes for the corresponding lead wires of the heating unit is prepared. Secondly, the heating unit is placed in the pot while pulling out the lead wires through the holes. Next, the lead wires are fastened to the holes with a sealer so that the heating element is positioned at the center of the pot and that the holes are completely sealed. Then, an epoxy series resin material is injected into the pot and hardened.
However, this conventional method requires minute work including pulling out the lead wired through the holes of the pot, positioning the heating element at the center of the pot using tweezers and fixing the lead wires to the holes with a sealer. In other words, complicated work is required to cover the heating unit with the electrical insulating covering material. Meanwhile, Japanese Publication for Examined Utility Model Application No. 9283/1991 does not disclose any method for solving the above-mentioned problems.
In a room with high humidity such as a bathroom, an anti-condensation mirror capable of preventing condensation from forming on the mirror by heating is conventionally used.
Japanese Publication for Unexamined Utility Model Application No. 155371/1985 discloses an anti-condensation mirror of this type. As described in the document, in the anti-condensation mirror, a plate-like heating element is attached to the rear surface of a mirror and the front surface of the mirror is heated by conducting electricity to the heating element. For example, the plate-like heating element is a film-like heating element formed by applying a thermal coating containing carbon and metal to a heat-resistant polymer film.
In the case of another anti-condensation mirror, a sheathed heater is attached to the rear surface of the mirror, and the front surface of the mirror is heated by conducting electricity to the sheathed heater. For example, the sheathed heater is a heating cable element formed by covering metallic wires with a heat-resistant polymer.
With these structure, however, in order to maintain the temperature of the heating element at a predetermined temperature and to ensure safety, it is necessary to provide a temperature control circuit and a circuit for preventing overheating. Consequently, the size of the anti-condensation mirror becomes larger. Additionally, when the film-like heating element is attached to the rear surface of the mirror, if a layer of air is produced between the film-like heating element and the mirror and if electricity is conducted to the heating element under this condition, there is a possibility of producing heat and causing fire. The reason for this is that the layer of air separates film-like heating element from the mirror at an area, and therefore the heat produced at the area can not escape, resulting in localized overheating. In the case of an anti-condensation mirror using the heating cable element, it is difficult to fasten the heating element closely to the rear surface of the mirror, resulting in low conductivity of the heat from the heating element to the mirror.
In order to overcome such difficulties, various types of anti-condensation mirrors incorporating a heater having a heating element made of the PTC thermistor as a heat source are suggested. With this structure, since the PTC thermistor has the self-controlling temperature characteristics, it is possible to omit the temperature control circuit and the circuit for preventing overheating, enabling a reduction in the size of the anti-condensation mirror. Moreover, there is no possibility that localized overheat causes a fire.
Japanese Publication for Unexamined Utility Model Application No. 108154/1989 discloses such a conventional-type anti-condensation mirror. This anti-condensation mirror is constructed by attaching a heater cable having a positive temperature coefficient of resistance to the periphery of the mirror and forming on the rear surface of the mirror a heat-transfer layer in contact with the heater. U.S. Pat. No. 4,933,533 also discloses a conventional-type anti-condensation mirror. This anti-condensation mirror is constructed by mounting a heating cable element on the rear surface of the mirror. The heating cable element is formed by covering a resin containing a carbon having a positive temperature coefficient of resistance with a polyvinyl chloride.
Furthermore, Japanese Publication for Unexamined Utility Model Application No. 65497/1973 also discloses such an anti-condensation mirror. This anti-condensation mirror is constructed as follows. An electrical conductive board, an electrical insulating substrate, an electrical conductive board and a thermal insulating board are mounted in this order on the rear surface of the mirror with or without a heat transfer board thereon. A PTC thermistor is inserted into each of a plurality through holes formed in the insulating substrate. The electrodes on both surfaces of the PTC thermistor are connected to both the conductive boards, so that electricity is conducted to the PTC thermistor through the conductive boards.
With this structure, in order to efficiently conduct the heat produced by the PTC thermistor to the mirror, it is necessary to provide a heat transfer board between the mirror and the PTC thermistor.
However, with the structure disclosed in the above Japanese Unexamined Utility Model Application No. 108154/1989, since the cable heater is attached to the periphery of the mirror, the anti-condensation effects are produced from the periphery. Consequently, if an anti-condensation mirror incorporates a large-sized mirror, it takes a longer time for a central area that usually requires anti-condensation effects to receive the effects.
With the structure disclosed in U.S. Pat. No. 4,933,533, the heating cable elements are pressed against the mirror by a plastic supporting member in order to bring the heating cable elements into contact directly with the rear surface of the mirror. However, as is disclosed in the same document, it is extremely difficult to attach the heating cable element of a considerably long length of 13.5 m to the mirror by evenly pressing it against the mirror. Moreover, since a space is formed between the mirror and the supporting member, it is difficult to efficiently and evenly conduct the heat from by the heating cable element to the mirror.
On the other hand, with the structure disclosed in the above Japanese Unexamined Utility Model Application No. 65497/1973, it is possible to solve the problems that Japanese Utility Model Application No. 108154/1989 and U.S. Pat. No. 4,933,533 have. More specifically, with this structure, since the PTC thermistor is mounted through the heat transfer and electrical conductive boards on a desired area of the rear surface of the mirror, it is possible to produce anti-condensation effects on the desired area of the mirror with a shorter time. However, since the heat transfer board, electrical conductive board, electrical insulating substrate, electrical conductive board and thermal insulating board are mounted on the rear surface of the mirror, the anti-condensation mirror has an increased thickness. This also causes increases in the size and weight of the anti-condensation mirror. In addition, since this anti-condensation mirror does not have an appropriate insulation structure, currents may leak.
Finally, each of the heating elements disclosed in the above-mentioned documents are designed without much considering the water vapor-proof properties of the anti-condensation mirrors when installed in a bathroom for example. With their structures, it is difficult to water and vapor-proof them. Therefore, when installing these anti-condensation mirrors in the bathroom, a voltage of commercial power supply can not be directly applied to the heating elements due to safety reasons. Namely, it is necessary to provide a transformer to lower the value of voltage of the power supply, for example, to a value not greater than 24 V, or to ask a specialized builder to install these anti-condensation mirrors. Thus, these structures result in increased costs and complicated handling of the anti-condensation mirrors.